Strong, defined abs don't require high-impact activity. Stay grounded on a mat or your own two feet and effectively train all the muscles of your belly. In 2001, the American Council on Exercise rated a number of ab exercises for their effectiveness. The best ones are all not only low-impact — they are no impact.
The basic crunch is a staple ab exercise that trains the rectus abdominis, the front sheath of your abs. It's inherently low-impact, but the basic version may not be the most impactful. Do the crunch with extended legs or on a stability ball for most effect.
Crunches alone don't target all of the muscles of the core. Planks train the deep transverse abdominis, which helps you stand tall and boosts balance. The classic plank has your torso suspended on your hands or forearms and toes.
Hold your body rigid, with the belly button pulled in toward the spine, for 20 to 90 seconds as a time. Once you master the basic version, add challenge with other options.